With the recent higher integration and larger capacity of large-scale integration (LSI) circuits, line widths of circuits desired for semiconductor devices have become even narrower. These semiconductor devices are manufactured, for example, in such a manner that, by using an original pattern (also referred to as a mask or a reticle; hereinafter comprehensively referred to as a mask) having a circuit pattern formed thereon, the pattern is exposed and transferred on a wafer by a reduction projection aligner referred to as a stepper, so as to form a circuit.
Improvement in yield is indispensable for the costly LSI manufacture. The yield is lowered when a defect in pattern exists in the mask to be used for transferring the pattern on semiconductor wafers. Thus, high accuracy of pattern inspection apparatuses for inspecting defects in masks for use in LSI manufacture is demanded.
In order to realize high accuracy of pattern inspection, illumination light rays with shorter wavelengths have come into use which achieve high resolution. For example, a deep ultraviolet ray of or below 200 nm in wavelength is used. Laser light, which is coherent light, is used as illumination light.
However, coherent light is highly interferential, and speckles (interference fringes) tend to occur, which adversely affects inspection accuracy. Hence, one method to reduce speckles is to pass laser light through a rotational phase plate (JP-A 2012-150036).